Poor bioavailability is a significant problem encountered in the development of therapeutic compositions, particularly those compounds containing a biologically active compound that is poorly soluble in water at physiological pH. An active compound's bioavailability is the degree to which the active compound becomes available to the target tissue in the body after systemic administration through, for example, oral or intravenous means. Many factors affect bioavailability, including the form of dosage and the solubility and dissolution rate of the active compound.
Poorly and slowly water-soluble compounds tend to be eliminated from the gastrointestinal tract before being absorbed into the circulation. In addition, poorly soluble active agents tend to be disfavored or even unsafe for intravenous administration due to the risk of particles of agent blocking blood flow through capillaries.
It is known that the rate of dissolution of a particulate drug will increase with increasing surface area. One way of increasing surface area is decreasing particle size. Consequently, methods of making finely divided or sized drugs have been studied with a view to controlling the size and size range of drug particles for pharmaceutical compositions.
For example, dry milling techniques have been used to reduce particle size and hence influence drug absorption. However, in conventional dry milling the limit of fineness is reached generally in the region of about 100 microns (100,000 nm), at which point material cakes on the milling chamber and prevents any further diminution of particle size. Alternatively, wet grinding may be employed to reduce particle size, but flocculation restricts the lower particle size limit to approximately 10 microns (10,000 nm). The wet milling process, however, is prone to contamination, thereby leading to a bias in the pharmaceutical art against wet milling. Another alternative milling technique, commercial airjet milling, has provided particles ranging in average size from as low as about 1 to about 50 microns (1,000-50,000 nm).
There are several approaches currently used to formulate poorly soluble active agents. One approach is to prepare the active agent as a soluble salt. Where this approach cannot be employed, alternate (usually physical) approaches are employed to improve the solubility of the active agent. Alternate approaches generally subject the active agent to physical conditions that change the agent's physical and or chemical properties to improve its solubility. These include process technologies such as micro-ionisation, modification of crystal or polymorphic structure, development of oil based solutions, use of co-solvents, surface stabilizers or complexing agents, micro-emulsions, super critical fluid and production of solid dispersions or solutions. More than one of these processes may be used in combination to improve formulation of a particular therapeutic compound.
These techniques for preparing such pharmaceutical compositions tend to be complex. By way of example, a principal technical difficulty encountered with emulsion polymerization is the removal of contaminants, such as unreacted monomers or initiators (which may have undesirable levels of toxicity), at the end of the manufacturing process.
Another method of providing reduced particle size is the formation of pharmaceutical drug microcapsules, which techniques include micronizing, polymerisation and co-dispersion. However, these techniques suffer from a number of disadvantages including at least the inability to produce sufficiently small particles such as those obtained by milling, and the presence of co-solvents and/or contaminants such as toxic monomers which are difficult to remove, leading to expensive manufacturing processes.
Over the last decade, intense scientific investigation has been carried out to improving the solubility of active agents by converting the agents to ultra fine powders by methods such as milling and grinding. These techniques may be used to increase the dissolution rate of a particulate solid by increasing the overall surface area and decreasing the average particle size.
U.S. Pat. No. 6,634,576 discloses examples of wet-milling a solid substrate, such as a pharmaceutically active compound, to produce a “synergetic co-mixture”.
International Patent Application PCT/AU2005/001977 (Nanoparticle Composition(s) and Method for Synthesis Thereof) describes, inter alia, a method comprising the step of contacting a precursor compound with a co-reactant under mechanochemical synthesis conditions wherein a solid-state chemical reaction between the precursor compound and the co-reactant produces therapeutically active nanoparticles dispersed in a carrier matrix. Mechanochemical synthesis, as discussed in International Patent Application PCT/AU2005/001977, refers to the use of mechanical energy to activate, initiate or promote a chemical reaction, a crystal structure transformation or a phase change in a material or a mixture of materials, for example by agitating a reaction mixture in the presence of a milling media to transfer mechanical energy to the reaction mixture, and includes without limitation “mechanochemical activation”, “mechanochemical processing”, “reactive milling”, and related processes.
The present invention provides methods for the preparation of biologically active compounds in nanoparticulate form, which ameliorate some of the problems attendant with prior technologies, or provides an alternative thereto.
As an example of the need for such novel compounds and methods for synthesizing them, consider osteoporosis. Osteoporosis describes a group of diseases which arises from diverse etiologies, but which are characterized by the net loss of bone mass per unit volume. The consequence of this loss of bone mass and resulting bone fracture is the failure of the skeleton to provide adequate support for the body. One of the most common types of osteoporosis is associated with menopause. Most women lose from about 20% to about 60% of the bone mass in the trabecular compartment of the bone within 3 to 6 years after the cessation of menses. This rapid loss is generally associated with an increase of bone resorption and formation. However, the resorptive cycle is more dominant and the result is a net loss of bone mass. Osteoporosis is a common and serious disease among postmenopausal women.
The most generally accepted method for the treatment of postmenopausal osteoporosis is estrogen replacement therapy. Although therapy is generally successful, patient compliance with the therapy is low, primarily because estrogen treatment frequently produces undesirable side effects. An additional method of treatment would be the administration of a bisphosphonate compound, such as, for example, Fosamax™ (Merck & Co., Inc.).
Throughout premenopausal time, most women have less incidence of cardiovascular disease than men of the same age. Following menopause, however, the rate of cardiovascular disease in women slowly increases to match the rate seen in men. This loss of protection has been linked to the loss of estrogen and, in particular, to the loss of estrogen's ability to regulate the levels of serum lipids. The nature of estrogen's ability to regulate serum lipids is not well understood, but evidence to date indicates that estrogen can up regulate the low density lipid (LDL) receptors in the liver to remove excess cholesterol. Additionally, estrogen appears to have some effect on the biosynthesis of cholesterol, and other beneficial effects on cardiovascular health.
It has been reported in the literature that serum lipid levels in postmenopausal women having estrogen replacement therapy return to concentrations found in the premenopausal state. Thus, estrogen would appear to be a reasonable treatment for this condition. However, the side effects of estrogen replacement therapy are not acceptable to many women, thus limiting the use of this therapy. An ideal therapy for this condition would be an agent which regulates serum lipid levels in a manner analogous to estrogen, but which is devoid of the side effects and risks associated with estrogen therapy.
A number of structurally unrelated compounds are capable of interacting with the estrogen receptor and producing unique in vivo profiles. Compounds with in vivo profiles typical of a “pure” antagonist (for example, ICI 164,384) or of a relatively “pure” agonist (for example, 17β-estradiol) represent opposite ends of a spectrum in this classification. Between these two extremes lie the SERMs (“selective estrogen receptor modulator”), characterized by clinical and/or preclinical selectivity as full or partial agonists in certain desired tissues (for example, bone), and antagonists or minimal agonists in reproductive tissues. Within this pharmacologic class, individual SERMs may be further differentiated based on profiles of activity in reproductive tissues.
Raloxifene, a second generation SERM, displays potentially useful selectivity in uterine tissue with apparent advantages over triphenylethylene-based estrogen receptor ligands. As such, raloxifene appears to be well-suited at least for the treatment of postmenopausal complications, including osteoporosis and cardiovascular disease. It is anticipated that, as further advances are made in the pharmacology and molecular biology of estrogen receptor active agents, further subclassifications of SERMs may evolve in the future along with an increased understanding of the therapeutic utility of these novel classes of estrogenic compounds.
The advancement of raloxifene has been hampered by its physical characteristics, particularly low solubility, which affects bioavailability. Accordingly, any improvement in the physical characteristics of raloxifene would potentially offer more beneficial therapies. In particular, it would be a significant contribution to the art to provide forms of raloxifene which have increased solubility, methods of preparation of such forms, pharmaceutical formulations comprising such forms, and methods of use of such formulations.
Although the background to the present invention is discussed in the context of improving the bioavailability of compounds that are poorly or slowly water soluble, the applications of the methods of the present invention are not limited to such, as is evident from the following description of the invention.
Further, although the background to the present invention is largely discussed in the context of improving the bioavailability of therapeutic or pharmaceutical compounds, the applications of the methods of the present invention are clearly not limited to such. For example, as is evident from the following description, applications of the methods of the present invention include but are not limited to: veterinary therapeutic applications and agricultural chemical applications, such as pesticide and herbicide applications.